When Sir Alex plays ­Hernandez, Rooney and Van Persie all ­together, that really is a frightening trio

Kevin Keegan

It was that combined strikeforce which the former Newcastle, Manchester City and ­Fulham boss could not shoot down in his ­managerial days.

They formed a goal-laden threesome – and Keegan still ­applauds them as the attacking ­formation which, more than anything, ­delivered the Champions League trophy to Old Trafford back in 2008.

But fast-forward four years and now Keegan sees danger signs again with Sir Alex ­Ferguson’s latest “hat-trick” of ­hitmen – just as powerful and equally as lethal as Rooney, Ronaldo and Tevez.

For those three, now substitute the fearsome formation of Rooney, Robin van Persie and Javier ­Hernandez. Keegan said: “When Sir Alex plays ­Hernandez, Rooney and Van Persie all ­together, that really is a frightening trio.

“One that rivals the Rooney-­Ronaldo-Tevez trinity which helped United lift the Champions League in 2008.

“Add in Danny Welbeck as well – an England international who is fourth-choice striker at the club – and you have an enviable set of attacking options.

“I managed teams against that Rooney-Ronaldo-Tevez ­combination and it was tough ­going. You could keep one, maybe two, quiet but you ran out of ­quality players to mark them.

“You didn’t know which one would provide the threat on any given day. But you were certain at least one would.

“And if it was a day when all three were on song – well you were in ­trouble. Hernandez looks full of ­confidence again when he plays, like the player who first broke into the United side two years ago.

“And he is scoring goals for fun. One of the problems when you are at a club like United is that when the quality runs so deep, there are always ­going to be world-class players sitting on the bench.

“Players who would find ­themselves walking straight into another team find themselves not playing for weeks on end.

“That’s how and why United stay at the top. That competition for places and hunger which exists between the players.

“Wayne and Robin will recognise that Hernandez is a serious threat to them. He’s not just some kid – he is a lad who is a real ­proven goalscorer who is now ­showing the ability he has when ­given the chance. And when they are all in there together, well as I have said, it’s frightening.”

There have been rumblings that Mexican ­Hernandez is getting ­frustrated with not ­playing a bigger part in the all-singing, ­all-dancing trio of ­talent on a weekly ­basis.

But Keegan is ­convinced that his once-fierce rival ­Ferguson will ensure that ­Hernandez enjoys a leading role ­alongside deadly Rooney and Van Persie.

He added: “It’s a tough job keeping all those ­players happy.

“And it means that ­Hernandez and Welbeck really must grasp any ­starting chances that ­happen to come their way with both hands.

“We have seen Hernandez at United and Edin Dzeko at City both given the ­‘supersub’ tag, thanks to their ability to score after coming on off the bench.

“And as a striker, I can ­honestly say that it is the last thing you want to be known as. The only way of getting rid of the label is to be doing the ­business from the start when given the chance.

“We have seen at City that ­although Dzeko is unhappy with being on the bench so often, there is just no ­denying that when he has started games he has not looked ­anything like he has when he’s come on.

“It will be interesting to see when Hernandez gets his chance and plays alongside Rooney and Van ­Persie.

“It’s not going to be a fun time for any ­defender when they get together on the pitch.

“It’s that goal threat which sees United starting to be the real deal again.

“And I think they are ­championship-winning ­material.

“United are ­traditionally better after Christmas, too.

“And as we all know, that is an ominous sign for City, Chelsea and the rest of the Premier League.”